Close and Latch Capability of Circuit Breaker (CB)

The close and latch capability of a circuit breaker is its ability to remain closed under short circuit fault situations. (OR). The close and latch current of a circuit breaker is the maximum current that the circuit breaker can safely remain closed under short circuit fault conditions.

For Power Frequency 60 Hz System:

For the 60 Hz system, the closing and latching current of the circuit breaker (CB) in kA peak is 2.6 times the rated RMS symmetrical short circuit current. The closing and latching current of the CB in kA RMS is 1.55 times the rated RMS symmetrical short circuit current.

The closing and latching current of the circuit breaker for the 60 Hz system is expressed in equations (1) and (2). Where, ISC is the rated RMS symmetrical short circuit current.

For Power Frequency 50 Hz System:

For the 50 Hz system, the closing and latching current of the circuit breaker (CB) in kA peak is 2.5 times the rated RMS symmetrical short circuit current. The closing and latching current of the CB in kA RMS is 1.47 times the rated RMS symmetrical short circuit current.

The closing and latching current of the circuit breaker for the 60 Hz system is expressed in equations (3) and (4). Where, ISC is the rated RMS symmetrical short circuit current.

References

• IEEE Std C37.04-2018 – IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Maximum Voltage Above 1000 V.

• IEEE Std C37.06-2009 – IEEE Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis–Preferred Ratings and Related Required Capabilities for Voltages Above 1000 V.

• C37.010-1999 Application Guide for AC High-Voltage Circuit Breakers.